Respiration rate monitoring of Antwerp treatment works showed periods when the activated sludge was up to 40% inhibited, with resultant sludge washout episodes. Using the Health monitoring test, a pattern of ‘normal’ respiration rates for a plant can be determined. Lower respiration rates will alert the treatment works to a chronic toxicity problem, allowing corrective actions to be taken. Toxicity monitoring can now be carried out on a routine daily basis, at all treatment works receiving potentially toxic wastewaters. This is especially important if there has been a history of poor plant performance.

Respiration rate of activated sludge has been recognised as a key controlling element in the modelling of process control, by the International Water Association Task Group. Actual respiration rate of the sludge in the aeration tanks, as well as the endogenous and maximum respiration rates are variables that indicate the rate of BOD removal and aeration requirements. The maximum respiration rate is also closely linked to the Critical Oxygen Concentration point, that is, the point at which diffusion over the bacterial cell walls ceases and therefore biodegradation is significantly compromised.

In industrial manufacturing companies, before a new process comes on-line, it is important to measure the toxicity of the effluent stream. This is especially the case for companies who treat their own waste. From the toxicity value (EC50) obtained, it may be a cost-effective solution to simply discharge the effluent to the treatment works at a slow and defined rate to minimise damage to the activated sludge. Following a period of acclimatisation it is often possible to increase feed rates to the plant and this can again be managed using the Strathtox Respirometer. An alternative approach is to undertake toxicity reduction procedures, such as neutralisation or acid hydrolysis, in pilot-scale laboratory studies. The Respiration or the Nitrification Inhibition Test may be used for this. In the future, toxicity reduction may be stipulated by local authority, water company or publicly-owned treatment works prior to giving discharge consents. Toxicity reduction may be evaluated using toxicity tests on activated sludge provided by the treatment works.

Water companies, water authorities or publicly-owned treatment works (POTW) need to have some knowledge of the composition of the wastes they it receive. In addition to testing for ammonia and BOD or COD levels, treatment works can license industrial discharges on the basis of concentrations of some of the known toxic compounds. However, it is recognised that very many non-regulated toxic materials still enter the treatment works and reduce the efficiency of biodegradation, and may cause toxic shock. The way is now open for more widespread use of direct toxicity tests as a basis for toxicity-based consents. Samples of the industrial effluent are collected at source, for testing on the actual bacteria of the receiving activated sludge. The tests used are the Respiration Inhibition Test and the Nitrification Inhibition Test. Note that this approach mirrors that of the regulators of discharges to receiving waters, who are now using direct toxicity tests (DTA) or whole effluent toxicity tests (WET tests) in order to protect the receiving environment.

If toxicity is identified in a mixed sewage entering a treatment works, the problem for the plant manager is to track and identify the source of the toxicity. This can be done by sampling the effluent stream at various points in its length. By working in a logical sequence back up the sewer network the number of samples can be minimised. This type of testing should be rigorously enforced for new product introduction by a waste producer, as well as sampling the effluents directly at source. The samples would be tested against the activated sludge of the receiving works, using the Respiration Inhibition Test or the Nitrification Inhibition Test.

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